Generally, the supply current is almost proportional to the voltage and frequency within a given operating voltage range. The current consists of two components: leakage current that flows even while the clock is stopped and switching current that depends on the clock frequency. As can be seen from the characteristics of a microcontroller in the STOP mode, the leakage current is negligibly lower than the switching current. The total supply current, therefore, is almost proportional to the frequency. Where the supply current is 10 mA at 5 V and 10 MHz, for example, it can be estimated that the current at 3 V and 5 MHz should be about 3 mA because 10 × (3/5) × (5/10) = 3.
In case of a microcontroller having an on-chip regulator, however, the current barely changes even if the supply voltage changes, because the regulator supplies a constant voltage to the internal circuits (except the clock generator and LCD controller). In this case, take only the frequency into consideration.